Stand-off control apparatus for plasma processing machines

ABSTRACT

A stand-off control apparatus for plasma processing machines which is capable of maintaining an optimum stand-off even when a processing speed varies. In a stand-off control apparatus for plasma processing machines which is provided with a microcomputer (4) for maintaining an optimum stand-off (h 0 ) by moving a torch toward/away from a work so that an independently inputted arc voltage (V) agrees with a speed-corrected target arc voltage (V O (F)) obtained by correcting a target arc voltage (V O ), which is stored in advance, for an independently inputted processing speed (F), the microcomputer (4) determines at the time of varying a processing speed (F) whether the speed is being accelerated, being decelerated or being kept unchanged, and provides that the speed-corrected target arc voltage (V O (F)) for the respective case is applied after the lapse of a corresponding delay time (Δt) which is independently stored.

TECHNICAL FIELD

The present invention relates to improvement in a stand-off controlapparatus for plasma processing machines, and particularly to astand-off control apparatus for plasma processing machines which iscapable of maintaining an optimum stand-off even when a cutting speedvaries.

BACKGROUND ART

A stand-off is the distance between a torch and a work to be processedin plasma processing. Maintaining the stand-off at an optimum value isselected as an objective in order to improve processing quality. Aconventional stand-off control apparatus for plasma processing machineswill be briefly described below.

A technique has been known (refer to, for example DE2706232) which triesto maintain an optimum stand-off h₀ by monitoring an arc voltage V fromthe fact that "a stand-off h is in proportion to the arc voltage V at afixed cutting speed F." However, the fixed stand-off system cannot copewith a case where the cutting speed F needs to be varied for improvedprocessing accuracy and productivity. In other words, the fixedstand-off system can manipulate two-dimensional processing machines likeXY tables with less difficulty, but basically cannot manipulatethree-dimensional processing machines, which cover a wider range ofcutting speeds.

In this connection, the applicants for the present invention havepreviously proposed a technique (refer to Japanese Patent ApplicationNo. 3-110790, published as Japanese Published Unexamined PatentApplication (A) 5-378) of maintaining the optimum stand-off h₀ bymonitoring the arc voltage and the processing speed F, wherein the factthat "the arc voltage V is substantially in inverse proportion to thecutting speed F", as shown in FIG. 5, is incorporated into the aforesaidtechnique. The proposed technique is hereinafter referred to asspeed-corrected stand-off system.

The correction of a target voltage V_(O) for a speed in thespeed-corrected stand-off system will now be described with reference toFIG. 5. In the figure, measurements of the cutting speed F and the arcvoltage V are plotted for stand-off's h₁ ˜h₅ (h₁ <h₅). For thestand-off's h₁ ˜h₅, as the cutting speed F increases, the arc voltage Vdrops substantially in an inversely proportional manner. This is becauseas the cutting speed F increases, the main anode point of the work comescloser to the torch. For example, with a reference cutting speed takenas F_(L) and a target arc voltage as V_(L) at the initially set optimumstand-off h₁, when the cutting speed increases to F_(H), the arc voltagedrops from V_(L) to V_(H). However, because of the target arc voltageV_(L) being fixed, the result of their comparison, i.e. V_(H) <V_(L),causes the torch to rise from h₁ to h₃ with a resultant failure tomaintain the optimum stand-off h₁. Hence, by correcting the initialtarget arc voltage V_(O) for an inputted cutting speed F, i.e., bymaking correction to a new target arc voltage V_(H) at the stand-off h₁and the cutting speed F_(H), the optimum stand-off h₁ is maintained evenwhen the cutting speed F varies. Accordingly, while the processing speedis kept unchanged at a stationary level before and after it varies, thequality of processing is quite good.

However, even in the speed-corrected stand-off system, when theprocessing speed varies, i.e., when variations in the processing speedbegin, are under way, and then end, it is difficult to maintain theoptimum stand-off h₀ because variations in the arc voltage V get delayeddue to delay in the response of the position of the plasma arc's mainanode point to operations of a processing machine. The time ofvariations in the processing speed is usually short, but even so, therearises a problem that the quality of processing deteriorates during thetime.

SUMMARY OF THE INVENTION

In view of the foregoing problem, it is an object of the presentinvention to provide a stand-off control apparatus for plasma processingmachines which is capable of maintaining an optimum stand-off even whena processing speed varies.

According to the present invention, in a stand-off control apparatus forplasma processing machines which is provided with a microcomputer formaintaining an optimum stand-off h₀ by moving a torch toward/away from awork to be processed so that an independently inputted arc voltage Vagrees with a speed-corrected target arc voltage V_(O)(F) obtained bycorrecting a target arc voltage V_(O), which is stored eitherautomatically or manually in advance, for an independently inputtedcutting speed F, the microcomputer determines at the time of varying acutting speed F whether the cutting speed F is being accelerated, beingdecelerated or being kept unchanged, and provides that thespeed-corrected target arc voltage V_(O)(F) for the respective case ofaccelerating the speed, decelerating the speed and keeping the speedunchanged is applied after the lapse of a corresponding delay time Δtwhich is independently stored for each of the cases.

Even when the cutting speed varies, the optimum stand-off h₀ can be keptunchanged by stabilizing the arc voltage V. The arc voltage V can bestabilized by preventing it from responding to a delay in operations ofthe processing machine and a delay in the response of a plasma arc.Hence, the microcomputer is designed to determine whether the cuttingspeed is being accelerated, being decelerated or being kept unchangedwhen the cutting speed varies. The delay time Δt corresponding to thedegree of a speed variation is stored in advance. Each delay time Δt isset so as to cover the delay in operations of the processing machine andthe delay in the response of the plasma arc. Accordingly, when themicrocomputer determines at the time of varying the cutting speed Fwhether the cutting speed F is being accelerated, being decelerated orbeing kept unchanged, it retrieves the delay time Δt corresponding tothe degree of the speed variation. After the lapse of the delay time Δt,the speed-corrected target arc voltage V_(O)(F) is applied. As a result,the arc voltage V is not influenced by the delay in operations of theprocessing machine and the delay in the response of the plasma arc,thereby maintaining the optimum stand-off h₀.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a stand-off control apparatus for plasmaprocessing machines according to an embodiment of the present invention;

FIG. 2 is a conceptual sketch of a plasma processing machine equippedwith the control apparatus according to the invention;

FIG. 3 is a flowchart of operations of a plasma processing machineincluding the control apparatus according to the invention;

FIG. 4 is a graph of characteristics comparing the invention in which adelay time is set and prior-art examples in which a delay time is notset; and

FIG. 5 is a graph showing the relationship between a processing speedand an arc voltage with stand-off as a parameter.

BEST MODE FOR CARRYING OUT THE INVENTION

A preferred embodiment of the present invention will now be describedwith reference to FIGS. 1-4. The present embodiment is an example ofapplying the present invention to a three-dimensionally driven plasmaprocessing machine of FIG. 2 and is associated with the flowchart ofoperations of a plasma processing machine of FIG. 3. Hence, thedescription of the present embodiment given below will also cover theplasma processing machine and the flowchart.

As shown in FIG. 2, a robot 5, which is the main body of the plasmaprocessing machine, substantially comprises a swing device 52 on a base51, a boom 53 mounted on the swing device 52, an arm 55 which is pinconnected with the boom 53 at its end and rises/lowers by means of ahydraulic actuator 54, a hand 56 which is pin connected with the arm 55at its end, and a microcomputer 4 which controls the robot 5. A torch 1is attached to the hand 56. An exhaust gas chute 6 is disposedunderneath the torch 1. A needle-point platform 61 is located at theupper portion of the exhaust gas chute 6, and a work 2 to be processedis placed on the needle-point platform 61 in such a manner as to beopposed to the torch 1. Plasma cutting is performed in the followingmanner. An arc voltage V is applied by a power source 41 between a torchelectrode 1a (negative pole) and the work 2 (positive pole) to beprocessed. An operation gas, fed separately to a nozzle 1b located atthe end of the torch 1, is converted to a plasma arc 3. The plasma arc3, together with an operation gas which is fed separately in such amanner as to enclose the plasma arc 3, is jetted to the work 2 to beprocessed on a cutting line, thereby melting the work 2 to be processedalong the line and blowing out molten splashes.

The microcomputer 4 to control the robot 5 comprises a robot controller57 and a stand-off controller 42. FIG. 1 shows a block diagram of themicrocomputer 4. Signals representative of a stable cutting speed F andan arc voltage V at an optimum stand-off h₀ immediately before startingthe cutting operation are inputted to the stand-off controller 42 fromthe robot controller 57, thereby automatically setting a referencecutting speed F_(O) and a target arc voltage V_(O) at the referencecutting speed F_(O) and the optimum stand-off h₀. The reference cuttingspeed F_(O) and the target arc voltage V_(o) can be set manually.

After processing has started, the stand-off controller 42 corrects thetarget arc voltage signal V_(O) for the cutting speed signal F which isperiodically inputted thereto from the robot controller 57, therebymaking a speed-corrected target arc voltage signal V_(O)(F). Accordingto the present embodiment, the cutting speed signal F and the arcvoltage signal V are inputted to the stand-off controller 42 from therobot controller 57. However, a speed sensor and a voltmeter may beprovided separately for inputting therefrom the cutting speed signal Fand the arc voltage signal V to the stand-off controller 42.

The stand-off controller 42 calculates variation in speed from theinputted cutting speed signal F to determine whether the cutting speed Fis being accelerated, being decelerated or being kept unchanged.According to the degree of a speed variation with respect to theaccelerated, decelerated or unchanged speed, the stand-off controller 42retrieves an adequate delay time Δt (Δt₁, Δt₂, Δt₃, . . . ), which isstored in advance for each case of accelerating the speed, deceleratingthe speed and keeping the speed unchanged. After the lapse of theretrieved delay time Δt, the stand-off controller 42 provides that thespeed-corrected target arc voltage signal V_(O)(F) is applied.

The steps of cutting the work 2 to be processed under the control of theplasma processing machine will hereinafter be described with referenceto FIG. 3. Of steps S1-S16 of FIG. 3, steps S6-S9 are additional stepsprovided by the present embodiment.

First, the robot controller 57 starts operating under an independentlyinputted cut instruction (S1), and positions the torch 1 to the optimumstand-off h₀ with respect to the work 2 to be processed (S2).

Next, the stand-off controller 42 achieves the stable cutting speed Fand the arc voltage V, inputted from the robot controller 57,immediately before starting the cutting operation by settingautomatically the reference cutting speed F_(O) and the target arcvoltage V_(O) at the reference cutting speed F_(O) and the optimumstand-off h₀, and instructing the robot controller 57 to start cutting(S3). The detailed description of the setting of the reference cuttingspeed F_(O) and the target arc voltage V_(O) is omitted because it isnot directly related to the present invention. A piercing operation orthe like can be conducted before starting the cutting operation in somecase, but the description thereof is also omitted because it is notdirectly related to the present invention.

Next, the stand-off controller 42 receives an actual cutting speedsignal F inputted from the robot controller 57 (S4), and corrects thetarget arc voltage signal V_(O) based on the actual cutting speed F toset the speed-corrected target arc voltage signal V_(O)(F) (S5).

Incidentally, when cutting approaches a corner or the like, the cuttingspeed F is decelerated, and then after cutting has passed the corner orthe like, the cutting speed F is accelerated. At such variation in thecutting speed F, the stand-off controller 42 determines whether thecutting speed F is being accelerated, being kept unchanged or beingdecelerated (S6). This determination is made by comparing a currentlyinputted cutting speed signal F and a previously inputted and storedcutting speed signal F₁.

If F>F₋₁, the cutting speed F is being accelerated; if F=F₋₁, thecutting speed F is being kept unchanged; and if F<F₋₁, the cutting speedF is being decelerated.

When the cutting speed F is determined to be accelerating, acorresponding delay time Δt₁ is retrieved (S8a); when the cutting speedF is determined to be unchanged, a corresponding delay time Δt₃ isretrieved; and when the cutting speed F is determined to bedecelerating, a corresponding delay time Δt₂ is retrieved (S8d). Then, aspeed-corrected target arc voltage signal V_(O{F)(t-αt)} obtained byshifting time by the delay time Δt₁, Δt₂ or Δt₃ is applied. Basically,these delay times Δt₁, Δt₂ and Δt₃ are set in consideration of the delayin operations of the plasma processing machine and the delay in theresponse of the plasma arc, and are stored in the stand-off controller42 in advance. In other words, with material of work, cutting speed,stand-off, arc voltage, and other factors taken into consideration, aplurality of delay times are set empirically, or the delay time is setin the manner of a linear function. With a special case of Δt₁ = Δt₂=Δt₃ taken also into consideration, preparation of at least one delaytime Δt will do in principle.

The aforesaid delay time Δt₃ for the case where the cutting speed F isdetermined to be unchanged does not appear in FIG. 3. The figure givesan example of processing by the use of only two pieces of delay time Δt₁and Δt₂. A case of the example is in the following construction. Whenthe cutting speed F is determined to be unchanged (S7), the followingthree cases are involved: a case where the cutting speed F is keptunchanged from the beginning, a case where the cutting speed F is keptunchanged after it has been accelerated, and a case where the cuttingspeed F is kept unchanged after it has been decelerated. These cases areredivided into the following two cases: a case where the cutting speed Fis kept unchanged from the beginning and a case where the cutting speedF is kept unchanged after it has been varied. Hence, in FIG. 3, it is amatter of course that the delay time Δt is not set when the cuttingspeed F is kept unchanged from the beginning (S8b), but rather when thecutting speed F is kept unchanged after it has been varied (S8c), thusthe speed-corrected target arc voltage signal V_(O{F)(t-Δt)} with thedelay time Δt₁ or Δt₂, set for acceleration or deceleration, is used(S9). Moreover, it is not shown, but when the cutting speed F is keptunchanged after it has been varied, a control cycle Δt₀ of the stand-offcontroller 42 can be replaced with the delay time Δt₁ or Δt₂.

Next, an actual arc voltage signal V is inputted (S10), and a deviationvoltage signal Δe=V_(O{F)(t-Δt)} is obtained from the difference betweenthe speed-corrected target arc voltage signal V_(O)(F) and the arcvoltage signal V (S11). Then, Δe is compared with zero (S12). If Δe>0,the torch is moved away from the work by a stand-off Δh corresponding tothe Δe (S13a); if Δe=0, the stand-off h₀ is maintained (S13b); and ifΔe<0, the torch is moved closer to the work by a stand-off Δhcorresponding to the Δe (S13c). V_(O{F)(t-Δt)} denotes a speed-correctedtarget arc voltage obtained after undergoing steps S6-S9, which areadditional steps provided by the present embodiment.

Steps S4-S13 are repeated based on a control cycle of the stand-offcontroller 42 until cutting is completed. When cutting is completed(S14), the stand-off controller 42 instructs the robot controller 57 toturn off the plasma and to move the torch away from the work (S15), andthen ends operation (S16).

FIG. 4 is a graph comparing the characteristics of prior-art examplesand the present embodiment. Characteristic values measured under thecondition that the stand-off is maintained fixed and that only thecutting speed is varied are plotted in the figure in the order of thecharacteristics of the prior fixed stand-off system (region A), thecharacteristics of the prior-art speed-corrected stand-off system(region B), and the experimental characteristics of the presentinvention (region C).

In regions A, B and C, a torch correction signal Δh, an arc voltage V, acutting speed correction term ΔF, and the existence/nonexistence of orthe state of input/output of the cutting speed F, from top to bottom,are plotted. Apart from the fixed stand-off system, the speed-correctedstand-off system and the present invention must be free of variations inthe torch correction signal Δh because the stand-off is held unchanged.This will hereinafter be described in detail.

In the fixed stand-off system (region A), the torch correction signalsΔh₁, Δh₂ emerge remarkably at acceleration (a₁ ˜a₂) and deceleration (a₃˜a₄) and even after a shift to the zone of keeping the cutting speedunchanged. This is a natural consequence of the fact that the fixedstand-off system does not make correction for speed and delay time. Tosuppress the torch correction signals Δh₁, Δh₂, the applicants for thepresent invention have previously proposed the speed-corrected stand-offsystem. However, even in the speed-corrected stand-off system (regionB), the torch correction signals Δh₃, Δh₄ emerge at acceleration (b₁˜b₂) and deceleration (b₃ ˜b₄) and even after a shift to the zone ofkeeping the cutting speed unchanged, even though the signals are lessintensive than the signals Δh₁, Δh₂. It is an object of the presentinvention to suppress the signals Δh₃, Δh₄. According to the presentinvention shown in region C, because of an additional delay timecorrection Δt, emergence of the torch correction signal Δh is suppressedgreatly at acceleration (c₁ ˜c₂) and deceleration (c₃ ˜c₄) and evenafter a shift to the zone of keeping the cutting speed unchanged. Thus,the torch correction signal is substantially in a flat state. Withoutquestion, the signal will be in a flatter state by selecting a moreappropriate delay time correction Δt. Noise caused by vibrations of therobot 5 may be responsible for vibrations of the torch correction signalΔh over the entire region. Hence, it may be possible to reduce thevibrations by controlling vibrations of the robot 5 and shielding anarithmetic circuit and the like from electromagnetic waves.

INDUSTRIAL APPLICABILITY

The present invention is effective in serving as a stand-off controlapparatus for plasma processing machines which is capable of securelymaintaining an optimum stand-off and providing excellent quality ofprocessing even when it is applied to three-dimensional robots and thelike.

What is claimed is:
 1. Apparatus for controlling a plasma processingmachine having a plasma torch and a device for moving said plasma torchtoward/away from a work to be processed, said apparatus comprising:meansfor providing a speed signal representative of an actual cutting speedof said plasma torch; means for providing an arc voltage signalrepresentative of an arc voltage between said plasma torch and saidwork; a microcomputer for storing a signal representative of a referencecutting speed of said plasma torch, a signal representative of a targetarc voltage at the reference cutting speed, and a plurality of delaytimes; said microcomputer having an input for receiving said speedsignal representative of an actual cutting speed of said plasma torch;said microcomputer having an input for receiving said arc voltage signalrepresentative of an arc voltage between said plasma torch and saidwork; said microcomputer being adapted to determine, in response to thethus received speed signal, variations in said actual cutting speed ofsaid plasma torch; said microcomputer being adapted to determine, inresponse to the thus determined variations, whether the actual cuttingspeed is being accelerated, is being decelerated or is being keptunchanged; said microcomputer being adapted to select a delay time fromsaid plurality of delay times in response to a determination as towhether the actual cutting speed is being accelerated, is beingdecelerated or is being kept unchanged; said microcomputer being adaptedto provide, after the thus selected delay time, a speed-corrected targetarc voltage signal by correcting said signal representative of saidtarget arc voltage for the thus determined variations in said actualcutting speed; and means for applying said speed-corrected target arcvoltage signal to said device to maintain an optimum stand-off betweensaid work and said plasma torch by moving said plasma torch toward/awayfrom said work so that after said selected delay time the actual arcvoltage agrees with said speed-corrected target arc voltage signal. 2.Apparatus in accordance with claim 1 wherein a first one of saidplurality of delay times corresponds to a determination that the actualcutting speed is being accelerated, and a second one of said pluralityof delay times corresponds to a determination that the actual cuttingspeed is being decelerated.
 3. Apparatus in accordance with claim 1wherein said device comprises a robot, and wherein said microcomputercomprises a robot controller and a stand-off controller.
 4. Apparatus inaccordance with claim 1, wherein said microcomputer is adapted todetermine said variations in said actual cutting speed by storing a thusreceived speed signal and comparing the thus stored speed signal with asubsequently received speed signal; and wherein said microcomputerdetermines, in response to the thus determined variations: that, whenthe subsequently received speed signal is greater than the stored speedsignal, the actual cutting speed is being accelerated; that, when thesubsequently received speed signal is less than the stored speed signal,the actual cutting speed is being decelerated; and that, when thesubsequently received speed signal equals the stored speed signal, theactual cutting speed is being kept unchanged.
 5. Apparatus comprising:aplasma processing machine having a plasma torch, a devices for movingsaid plasma torch toward/away from a work to be processed and forproviding a relative movement between said plasma torch and said workalong a cutting line at a cutting speed, and a power source forproviding an arc voltage between said plasma torch and said work; adevice controller for controlling said device to move said plasma torchtoward/away from said work; a stand-off controller for storing a signalrepresentative of a reference cutting speed of said plasma torch, asignal representative of a target arc voltage at the reference cuttingspeed, and a plurality of delay times; means for inputting to saidstand-off controller a speed signal representative of the actual cuttingspeed of said plasma torch; means for inputting to said stand-offcontroller an arc voltage signal representative of the actual arcvoltage of said plasma torch; wherein said stand-off controllerdetermines a variation in the thus inputted speed signal; determineswhether the actual cutting speed is being accelerated, is beingdecelerated or is being kept unchanged; selects a delay time from saidplurality of delay times in response to a determination as to whetherthe actual cutting speed is being accelerated, is being decelerated oris being kept unchanged; and after the thus selected delay time providesa corrected signal representative of a speed-corrected target arcvoltage; and means for applying said corrected signal to said devicecontroller to maintain an optimum stand-off between said work and saidplasma torch by moving said plasma torch toward/away from said work sothat after said selected delay time said actual arc voltage agrees withsaid speed-corrected target arc voltage, said speed-corrected target arcvoltage being a correction of said target arc voltage for the thusdetermined variation in said actual cutting speed.
 6. Apparatus inaccordance with claim 1, wherein a first one of said plurality of delaytimes corresponds to a determination that the actual cutting speed isbeing accelerated, and a second one of said plurality of delay timescorresponds to a determination that the actual cutting speed is beingdecelerated.
 7. Apparatus in accordance with claim 6, wherein saidstand-off controller is adapted to determine said variation in said thusinputted actual cutting speed by storing a thus inputted received speedand comparing the thus stored speed with a subsequently inputted actualcutting speed; and wherein said stand-off controller determines, inresponse to the thus determined variation: that, when the subsequentlyinputted actual cutting speed is greater than the stored speed, theactual cutting speed is being accelerated; that, when the subsequentlyinputted actual cutting speed is less than the stored speed, the actualcutting speed is being decelerated; and that, when the subsequentlyinputted actual cutting speed equals the stored speed, the actualcutting speed is being kept unchanged.
 8. A method comprising the stepsof:positioning a plasma torch at a stand-off distance from a work to becut by the plasma torch; storing a signal representative of a referencecutting speed of said plasma torch, a signal representative of a targetarc voltage at the reference cutting speed, and a plurality of delaytimes; establishing an actual arc voltage between said plasma torch andsaid work to form a plasma arc; providing a relative movement betweensaid plasma torch and said work along a cutting line at a cutting speedso that said plasma arc cuts said work along said cutting line;ascertaining an actual cutting speed of said plasma torch; ascertainingan actual arc voltage of said plasma torch; determining a variation insaid actual cutting speed; determining, from the thus determinedvariation in said actual cutting speed, whether said actual cuttingspeed is being accelerated, is being decelerated or is being keptunchanged; selecting a delay time from said plurality of delay times inresponse to a determination as to whether said actual cutting speed isbeing accelerated, is being decelerated or is being kept unchanged; andafter the thus selected delay time moving said plasma torch toward/awayfrom said work to vary said stand-off distance in response to the thusdetermined variation in said actual cutting speed so as to maintain anoptimum stand-off distance between said work and said plasma torch.
 9. Amethod in accordance with claim 8, wherein said step of selecting adelay time comprises selecting a first one of said plurality of delaytimes when the actual cutting speed is being accelerated, and selectinga second one of said plurality of delay times when the actual cuttingspeed is being decelerated.
 10. A method in accordance with claim 8,wherein said step of moving said plasma torch toward/away from said workto vary said stand-off distance comprises:modifying said signalrepresentative of said target arc voltage responsive to the thusdetermined variation in said actual cutting speed to establish a signalrepresentative of a speed-corrected target arc voltage; and, after saidselected delay time, moving said plasma torch toward/away from said workto vary said stand-off distance in response to said signalrepresentative of said speed-corrected target arc voltage so that saidactual arc voltage agrees with said speed-corrected target arc voltage.11. A method in accordance with claim 10, wherein said step of selectinga delay time comprises selecting a first one of said plurality of delaytimes when the actual cutting speed is being accelerated, and selectinga second one of said plurality of delay times when the actual cuttingspeed is being decelerated.
 12. A method in accordance with claim 11,wherein said step of determining a variation in said actual cuttingspeed comprises storing a first signal representative of a thusascertained actual arc voltage, and comparing the thus stored firstsignal with a second signal representative of a subsequently ascertainedactual arc voltage; andwherein said step of determining, from the thusdetermined variation in said actual cutting speed, comprises determiningthat, when the second speed signal is greater than the first signal, theactual cutting speed is being accelerated; that, when the second signalis less than the first signal, the actual cutting speed is beingdecelerated; and that, when the second signal equals the first signal,the actual cutting speed is being kept unchanged.
 13. A method inaccordance with claim 8, wherein said step of determining a variation insaid actual cutting speed comprises storing a first signalrepresentative of a thus ascertained actual arc voltage, and comparingthe thus stored first signal with a second signal representative of asubsequently ascertained actual arc voltage; andwherein said step ofdetermining, from the thus determined variation in said actual cuttingspeed, comprises determining that, when the second speed signal isgreater than the first signal, the actual cutting speed is beingaccelerated; that, when the second signal is less than the first signal,the actual cutting speed is being decelerated; and that, when the secondsignal equals the first signal, the actual cutting speed is being keptunchanged.